Glossary

A

Acknowledge

A message sent by the intersection manager to driver agents to
acknowledge the receipt of a Cancel or Reservation-Completed message. Once a driver agent has
received this message after sending a Cancel
message, it knows that the reservation it held previously has been
cancelled and it is free to send a new Request
message.

All-Lanes

A light model in which all the lights for all lanes in each
direction are synchronized. If there are four directions of traffic -
north, east, south, west - then the light model would dictate that all
northbound lanes be given a green light simultaneously, followed by all
eastbound lanes, southbound lanes, westbound lanes, and then northbound
lanes and so forth. This light model is a good fit for scenarios
involving many human drivers because green lights are frequent and it
behaves very similarly to modern-day traffic lights.

Autonomous Vehicle

A vehicle which does not require a human driver. Autonomous
vehicles are controlled by driver agents.

C

Cancel

A message sent by driver agents to intersection managers
indicating that the driver agent would like to cancel the reservation it
is currently holding.

Change-Request

A message sent by driver agents to intersection managers indicating
that the driver agent would like to change the parameters of the
reservation it is currently holding.

Confirm

A message sent by intersection managers to driver agents confirming
a reservation. This can be in response to either a Request message or a Change-Request message. In the latter case, it
indicates that the new reservation parameters are acceptable and the old
reservation has been canceled.

D

Driver Agent

A computer program responsible for driving an autonomous vehicle.
The driver agent is one of two classes of autonomous agents that make up
the reservation system.

Delay

The difference between the amount of time it takes a vehicle to
reach its destination and the time it would take to reach the
destination under optimal conditions.

Done

A message sent by driver agents to intersection managers indicating
that the vehicle controlled by the driver agent has safely completed its
journey across the intersection. This message can also be used to
report useful statistics back to the intersection manager. This message
was previously named Reservation-Completed, but
was renamed for simplicity and brevity.

F

FCFS

The "First Come, First Served" intersection control policy. This
policy divides the intersection into an grid of tiles. To determine if
reservation requests can be granted, the policy simulates the trajectory
of the vehicle through the intersection. If, at every time step of the
simulation, the simulated vehicle does not occupy any previously
reserved tiles, the reservation is granted. Otherwise, it is rejected.
This policy incurs the lowest delays to vehicles, but requires that all
vehicles be autonomous and able to interact with the reservation
system.

FCFS-Emerg

A version of the FCFS policy which gives
preference to emergency vehicles. Once a Request or Change-Request
message from an emergency vehicle is received, FCFS-Emerg rejects all reservation requests from lanes
that do not contain emergency vehicles. The policy returns to the
default FCFS behavior when all emergency
vehicles successfully cross the intersection.

FCFS-Light

A version of the FCFS policy that can be
used by human drivers. Associated with each instance of the policy is a
light model. When FCFS-Light receives requests,
it first determines if the lane in which the requesting driver agent is
in will have a green light at the requested arrival time. If so, the
reservation is confirmed. If the light will be yellow, the reservation is
rejected. If the light will be red, the reservation parameters are
passed to the FCFS portion of the policy which
checks its database of reserved tiles as well as the off-limits tiles to
determine whether or confirm or reject the reservation.

G

Granularity

For the FCFS policies, the number of tiles
on a side of the square grid in which the intersection is divided.
Thus, if the policy divides the intersection into 24 × 24 = 576
tiles, the granularity of the policy would be 24.

I

Intersection Control Policy

The part of an intersection manager agent that determines whether or
not to grant reservation requests. Some examples are FCFS, Stop-Sign, and Traffic-Light.

Intersection Manager

An agent that coordinates autonomous vehicles at an intersection. An
intersection manager may also control the traffic lights at
an intersection, in order to communicate with humans. Intersection
managers receive reservation request messages (along with other types of
messages) from driver agents and, using an intersection control policy, decide
whether or not to grant these requests. The intersection manager is one
of two classes of autonomous agents that make up the reservation
system.

L

Light Model

A component of the FCFS-Light intersection
control policy. The light model serves two purposes. First, it
physically controls the traffic lights at the intersection. Second, it
provides information to the intersection control policy about the state
of the lights at any time. The policy may then use this information to
make decisions regarding reservation requests.

O

Off-Limits Tiles

For the FCFS-Light policy, the tiles that
are reserved for vehicles operating under the control of the light
model. For example, if the light for the northbound left turn lane is
green, then all tiles that could be used by a vehicle turning left from
that lane are off-limits.

Overpass

An intersection control policy used to represent optimal
intersection control. This policy would never be used because it makes
no safety guarantees. However, in simulation it performs well as a
benchmark policy.

R

Reject

A message sent by intersection managers to driver agents indicating
that the reservation request in a Request or Change-Request message was not acceptable. The
message also may include a reason for why the request could not be
granted.

Request

A message sent by driver agents to intersection managers requesting
a reservation. The message contains the parameters the intersection
manager would need in order to determine if the reservation can safely be
granted.

Reservation

A set of parameters describing a vehicle's trajectory through an intersection.

Reservation-Completed

See Done

Reservation System

A multiagent system composed of two different classes of agents:
driver agents and intersection managers. The driver agents "call ahead"
to the intersection manager in an attempt to resreve space-time in the
intersection. The intersection managers are responsible for confirming
or rejecting these requests in a manner that prevents vehicles from
colliding in the intersection.

S

Simulator

The custom time-based program we have written in order to run
experiments on the reservation system. The program is written in Java.

Single-Lane

A light model in which green lights are given to only one lane at a
time. For example, the left turn lane of the northbound traffic would
have a green light, while all other lanes would have a red light. Next,
the straight lane of the northbound traffic would have a green light,
then the right turn. Next, the green light would go through each lane
of eastbound traffic, and so forth.

Spawn probability

The probability that at any time step of simulation the simulator
will create ("spawn") a new vehicle driving towards the intersection.
Even when this happens, a spawn may be aborted if there is no safe place
to create the vehicle. This probability is used to control the amount
of traffic in an experiment.

Stop-Sign

An intersection control policy that approximates a modern day stop
sign. The policy rejects all requests that specify an arrival time that
is not the current time. If the arrival time in the request is the
current time, the behavior defaults to the FCFS
policy. Because it can more precisely determine if vehicles are safe to
proceed, it performs better than an actual stop sign with human drivers
would. However, the policy could conceivably be used if there were a
reason to bring all traffic to a stop before crossing the intersection
(e.g. road construction or a downed power line).

T

Tile

A representation of a portion of the intersection used by FCFS policies. The policies record reservation
information for each tile and then use that information to decide
whether or not to grant future requests by vehicles.

Traffic-Light

An intersection control policy that emulates a modern-day
traffic light. By granting reservations only when the corresponding
lights would be green, vehicles are controlled in a manner identical to
traffic lights.